The U.S.-French ocean topography
satellite, known as the TOPEX/Poseidon mission and scheduled for launch in
mid-July, will be able to study global weather conditions like the El Niņo that
brought heavy rains to Southern California this month.

The weather abnormality, caused by an
anomalous warming of the surface water throughout the central equatorial
Pacific Ocean, occurs about every four to seven years, usually beginning in
December.

The El Niņo that we are having warms
the Pacific Ocean along a region of about 240 kilometers (150 miles) north and
south of the equator, said Dr. David Halpern, an oceanographer and senior
research scientist at JPL. El Niņos typically warm the equatorial waters
by about 2 to 4 degrees Centigrade (4 to 7 degrees Fahrenheit).

Sometimes it produces the heavy
rains, but not always, he said. The last time an El Niņo occurred,
in 1987, it did not influence our rains in Southern California.

While El Niņos do not always bring storm
fronts to Southern California, Halpern said, they are almost always associated
with seasons of intense rainfall.

Most of the clouds in the Pacific
develop in the western tropical Pacific, where the water is generally quite
warm, Halpern said. They follow a path -- known as Pineapple Alley
-- from the Hawaiian Islands into Washington and Oregon.

The other source of storms in Southern
California comes from the Gulf of Alaska and heads southward, usually veering
toward the Pacific Northwest.

During an El Niņo, these storm
patterns along Pineapple Alley and from the Alaskan gulf are changed, moving
about 1,100 to 1,600 kilometers (700 to 1,000 miles) south so that they now
fall over Southern California rather than in Oregon and Washington, said
Halpern, the first to observe the El Niņo ocean current along the equator.

Winds are the driving force behind
ocean weather, he said. The ocean warming in the equatorial Pacific west
of the date line is maintained by westward-blowing trade winds. Every several
years, the strength of these winds diminishes and sometimes reverses direction.

This creates a massive flow of warm
water into the central and eastern equatorial Pacific, where the surface waters
are normally much cooler than west of the date line, he said.

Changes in sea-surface temperature in the
Pacific equatorial waters occur at irregular intervals and in conjunction with
the southern oscillation -- the seesawing fluctuation of
atmospheric pressure between the eastern and the western South Pacific.

Because this phenomenon occurs in one of
the most sparsely populated regions of the world where very few observations
are made, it was not linked to global weather patterns, including the United
States, until quite recently.

Oceanographers and meteorologists have
traditionally relied on National Oceanic and Atmospheric Administration (NOAA)
satellites to learn more about these ocean-atmospheric interactions in the
Pacific Ocean.

Methods for measuring global surface winds
-- a very important dimension of sea-surface temperature fluctuations
associated with El Niņo s -- were introduced by an instrument called a
scatterometer on NASA-JPL's 1978 Seasat experimental satellite. The instrument
-- which used radar to measure ocean winds -- was the granddaddy of a
generation of scatterometers under development at JPL.

The U.S.-French TOPEX/Poseidon mission
will provide another dimension to studies of ocean circulation.

In order to better predict El Niņos,
we have to understand something about how the temperature is changed within the
interior of the ocean, Halpern said. The TOPEX altimeter will allow
us to measure the sea-surface height in the equatorial Pacific, where only a
few islands exist to record sea level.

TOPEX will be able to estimate how much the
water warms from the surface to about 100 meters (328 feet) below the surface
of the ocean. By measuring the rise of the equatorial sea surface, scientists
will know how much warm water has been introduced by eastward-flowing currents
in the upper ocean.

Halpern, a recent visiting professor at
Caltech, will be able to improve predictions of the El Niņo currents using
TOPEX data. The surface oceanographic data from satellite radar altimeters,
scatterometers and radiometers will be assimilated into time-dependent,
three-dimensional ocean general circulation models with advanced supercomputers
to produce realistic descriptions of the subsurface current and temperature
conditions.

Halpern is chairman of the Pacific Ocean
Panel of the Committee on Climate Changes and the Ocean (CCCO) and a member of
the National Academy of Science's Advisory Panel on TOGA (Tropical Ocean Global
Atmosphere). His research is conducted for the Climate and Hydrologic System
branch of NASA's Office of Space Science and Applications. The branch is
directed by Dr. Robert Schiffer.

The U.S. portion of the TOPEX/Poseidon
mission is managed by JPL's Charles Yamarone, project manager, and Dr.
Lee-Lueng Fu, project scientist, for NASA's Office of Space Science and
Applications.